Your feet are made to roll as you move across them in a forward or backward direction, which is why we take heel leads on normal forward steps and toe releases on backward steps. When stepping sideways, however, you move off of and arrive first on toes, cushioning your arrival over the foot. Now between forward and side is a sizable range of diagonal movement, the majority of which, from the standpoint of foot mechanics, falls under the realm of the side step. In other words, a step has to be pretty close to straight forward, perhaps forward and slightly side, to warrant a heel lead/toe release. Once you approach diagonally forward, the body naturally abandons the rolling action in favor of toe cushioning. You can still force a heel, but its at this point that a rolling foot stops being advantageous, as it is no longer rolling in the direction of the body weight.

In Tango, the forward and back steps mostly fall within the range of what our bodies should interpret as heel lead/toe release. The "side leading" shouldn't change this fact, save one exception: The man's LF backward step with a left side leading, which according to the ISTD, should be taken with the right toe maintaining contact with the floor (i.e. a heel release).

I've spoken with many top ranking pros about the reason for this exception, because I agree with you, it does "feel right". But it's an interesting point to consider: Why the man only, and not the lady? Of all of the people I've spoken to, including a few world finalists and at least one who has studied "old school" with some of the biggest name of the previous generations, there has been no absolute consensus -- only theories. Some of them are decent, however. Here are two of the best:

(1) Whenever it comes to matters of footwork differences between man and lady, you should look to two sources: The difference in poise and the difference in heel height. In the case of the LF back walk in Tango, the former is more convincing. Her backward and leftward poise sends her across the heel slightly earlier on the previous step, therefore to release the toe on the next.

(2) Although he ISTD book may describe this step for both man and lady identically -- "LF back, left side leading" -- It is not in fact an identical direction in actual practice. If you think of the circumstances in which you find this step, for the lady it's usually a true basic curving backward walk. How often is the man taking a series of basic curving backward walks? His version of "LF back, left side leading" is typically found in figures such as the Back Corte, where one tends to dance this walk with a bit more of a sideways component... enough to affect the mechanics of the foot.